The present invention relates to a method for generating hydrogen and/or synthesis gas by steam-hydrocarbon reforming along with generating steam using waste heat from the steam-hydrocarbon reforming process. More particularly, the present invention relates to such a process with little or no steam export.
Synthesis gas is used to produce products such as ammonia, methanol and hydrogen. Synthesis gas is generated by high temperature processes where a lot of waste heat is available. The waste heat is generally used to generate steam and helps to improve the overall efficiency of the synthesis gas facility. In typical facilities, the amount of steam generated from the waste heat significantly exceeds the amount of steam needed for reforming of a hydrocarbon feed in a steam-hydrocarbon reformer. The excess steam is exported or used to generate power in a steam turbine.
However, exporting steam requires expensive pipeline systems including control and safety valves, steam traps, heat tracing, etc. Exporting steam is justified when steam is needed nearby and/or a customer is willing to pay a reasonable price for the steam. Exporting steam can also impose constraints on plant location in order to minimize the length of the steam export piping.
Facilities for producing synthesis gas generate large amounts of steam from the waste heat. Depending on the design, overall steam production may be 35% to 300% more than required for internal use in the steam-hydrocarbon reformer. Current industry practice is to export the excess steam or to use the steam in a steam turbine for power production. Both options require additional capital expenditure and can be cost prohibitive for projects where there is no customer willing to buy the steam at a reasonable cost, or power can not be produced competitively.
For small hydrogen production units where steam export is not justified, a portion of the excess steam is often used in the process less efficiently or vented. The hydrogen plant may be designed with less heat recovery equipment resulting in a less efficient plant.
There are a number of design options that have been used to vary the total steam production from the synthesis gas plant and reduce steam export. These design options take into account process limitations such as supplemental fuel requirements for the catalytic steam reformer.
One widely used option is to preheat the combustion air for use in the reformer to high temperature, for example up to 600° C. (1100° F.). Combustion air is typically preheated in the convection section of the reformer and can be arranged using one or two stages depending on the desired preheat temperature. Preheating the combustion air helps to reduce the amount of fuel required for combustion in the reformer. Since less fuel is used, the flow of flue gases from the reformer is reduced resulting in less waste heat for generating steam.
Fuel preheating has a similar but smaller impact on overall steam production.
Another option is to use an adiabatic prereformer. An adiabatic prereformer is a vessel filled with Nickel-based reforming catalyst that is located upstream of the primary reformer. A mixed feed of steam and a hydrocarbon are fed to the adiabatic prereformer at a high temperature. The prereformed product is heated again by the combustion product gases and then fed to the primary reformer.
Use of a prereformer allows the recovery of additional heat from the flue gas back to the process by heating the prereformer effluent stream, thus reducing the amount of heat available for steam generation. Use of a prereformer has other benefits such as removing higher hydrocarbons from the feed stream to the primary reformer.
Facilities including a prereformer are typically cost effective since the size of the primary reformer may be reduced while maintaining high efficiency.
These methods to reduce the amount of steam are useful for cases where export steam has little or no value. However, the use of air preheat and/or a prereformer still has significant waste heat and can not provide suitable efficiencies without taking credit for steam export.
When credit for the steam produced can not be reasonably factored in to the efficiency of the synthesis gas generating facility, methods are required to lessen the impact on plant efficiency.
There is a need to lessen the impact on plant efficiency when little or no export steam is needed or produced. It would be desirable to produce hydrogen in a reforming process while producing little or no export steam and while maintaining overall plant efficiency.